The invention relates to a device for determining the water content of a target having at least one source of target radiation and at least one source of reference radiation, both directed to the surface of the target, as well as at least one detecting element for measuring the intensity of the radiation reflected back by the target surface.
Such contactless determination of a water content can be conducted for any target. Such water content determination is particularly but not exclusively required in the manufacture of paper.
In particular, the invention relates to devices for determining the water content of the material mix of water and fibrous materials from which paper is prepared by continuous dehydration.
The water content determination can find use in the exploitation of old fibers or in connection with the process of slurry-pressing from wastewater residues, where the water content of a web of fibrous material mounted on dry sieves is monitored.
According to German published patent application DE 31 49 869 A1, a device for measuring the humidity of a paper web is already known; in the device, two light-emitting diodes emit an alternating pulse frequency of infrared-light pulses with differing wavelengths, and the intensity of the reflected radiation is measured. The wavelength of one of the two light pulses corresponds to the absorbance line in the IR absorbance spectrum of water, while the wavelength of the other light pulse is clearly different from that of the absorbance wavelength and serves as a reference signal. When maintaining the proportion of the intensities of target and reference beams at a constant level, the reflected radiation intensities of both wavelengths are measured, and the water content is determined on that basis. Focusing and reflecting the beam, which is required for measurement, involve a relatively high constructive and technical effort. During operation, device components present in the course of the optical path of the target and reference beams as well as that of the reflected beam are relatively easily contaminated.
In areas of paper manufacturing equipment in which the paper web has a very high water content, the respective environment conditions have not allowed functional measurement so far, as liquid droplets present in the air and other suspended particles result in an immediate blockade of the measurement optics and other optical elements in conventional measurement devices, making operation over an extended period of time impossible.
In other known devices, target and reference beams as well as the reflected radiation are led from and/or to a measurement device through fiber-optic elements. Again, the relatively high technical effort constitutes a disadvantage here, resulting in an increased failure probability and allowing limited measurement sensibility only.